1. Field of the Invention
The present invention relates to a high-speed motor, such as brushless motors, reluctance type motors, stepping motors, and DC motors, which are required to be capable of rotating in a high speed region without causing fall of output torque.
2. Description of the Related Art
Conventionally, a reluctance type motor has so many disadvantages that few reluctance type motor have ever been practically utilized, regardless of its advantages such as large output torque and no need of magnet rotor. A utilization of a stepping motor, having a large output, is restricted only to a special purpose because of its slow stepping speed. Although high-speed DC or brushless motors have been available, they have never widely been used because of their low efficiency in a high-speed region.
The first problem of the motor according to the above-described conventional prior art lies in that it requires respective armature coils to be controlled by switching elements connected to both ends thereof for activating or deactivating these armature coils, necessitating to use a number of expensive power elements, which entails the increase of manufacturing cost.
Furthermore, switching elements provided at a positive terminal side of an electric power source require input electric signals to be supplied from another electric power source for controlling currents supplied to the armature coils, thereby contributing to the increase of manufacturing cost.
The second problem of a reluctance type motor rises from that it has a rotor equipped with numerous salient poles, thereby causing large inductance; this further causes increases of magnetic energy amount stored into or discharged from magnetic poles or salient poles and increase in repetition frequency of such energy storage and discharge during one complete revolution of the rotor, which eventually leads to a problem that the reluctance type motor is unable to rotate in a high-speed region regardless of its large output torque. Also, in the case of a DC motor, similar problem would occur if required to rotate at a higher speed. Here, a low speed should be referred to as around 300 r.p.m, and a high speed as around 20 thousands rpm.
The third problem rises from that, in the case of a motor having a large output, an extraordinarily large inductance of the armature coil will cause a slow building-up of exciting current at an initial stage of the current supply period, as well as from a slow trailing-edge at a terminating stage of the current supply period. The former will cause a lower output torque, and the latter a counter torque.
If power source voltage is increased in order to rapidly building up armature current in the initial stage of the current supply period, the armature current will build up sharply after the magnetic saturation point. This will cause vibrations and electric noises of the motor, thereby adding to the disadvantages coupled with the fact that the above-described building-up section of the armature current corresponds to a section where the torque is small.
In other words, the problem lies in that a high-speed rotation (i.e. several tens of thousands rpm) cannot be realized due to above-described fall of torque and occurrence of counter torque. Even if the rotational speed is reduced to a generally used speed region (i.e. several thousands rpm), a significant amount of torque fall and counter torque still occur, thereby causing the fall of the efficiency of a motor. A possible measure for increasing a rotational speed will require an electric power source voltage to be increased up to 1000 volts or more, which will lack in practical utility.
Finally, the fourth problem would be that a solution of the first problem by providing one switching element only at a negative terminal side of the armature coil may render the circuit configuration complicated, because an additional circuit will be required for reducing charge/discharge time of magnetic energy in the armature coil.